Hydraulic valve tappet



Aug. 20, 1957 B. c. lSKINNER 2,803,231

HYDRAULIC VALVE-TAPPET Filed nec. 1, 195s INTOR BY BPO/wmv 0MM/fe United States HYDRAULIC VALVE TAPPET Bronson C. Skinner, Dunedin, Fla., assigner to Brunhilde W. Skinner, Dunedin, Fla.

This invention relates to simple, novel, practical and etective hydraulic valve tappets, designed to be interposed between actuating cams on internal combustion engine cam shafts and the ends of valve stems or push rods which operate engine valves, and more particularly, of the type disclosed and claimed in the copending applicationof Robert C. Moser, Serial No. 331,290, filed .'zanuary 14, 1953.

A general object of the present invention is to provide such a valve tappet in which means are provided which minimize collect-ion of gaseous medium, such as oilentrained air, in the valved compression -oil chamber thereof and avoid gas lock therein, consequentially avoiding etectively a spongy or too springy `action in lifting an engine valve so as to assure maintenance of perfect timing, while minimizing loss of oil to an unusual extent.

A more specific object of the invention is the provision in such a valve tappet of means for venting collected gaseous medium effectively from the valved -compression chamber without undue oil loss.

A. further object of the invention is to provide for such purpose a small venting hole or bleed duct leading from the compression chamber inlet passage through the piston inward of the valve, and from any pocket associated therewith, out to the space between the shell wall and pistonfor effectiveV escape of gaseous medium.

Other objectsof the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangement of4 parts, whichl will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in theclaims.

For a fuller understanding of the nature and objects ofthe invention reference should be had to the following detaileddescription taken in connection with the accompanying drawing, in which:

Fig. l is a logitudinal vertical section through a tappet comprising one form. of my invention, showing a fragmentary portion of the :guide for the tappet in axial section and a portion of `a push rod in elevation, and with parts in the-positionwhichithey take as the actuating cam therefor moves from base circle contact of the cam with the tappet to the lifting position of the cam, this being when the tappetis about to be immediately lifted to open an engine valve;

Fig.v 2 isan axial section through a modiiied form of tappet. of the present invention, and showing in elevation a portion of the-push rod;

Fig. 3 is an axialsection similar to Fig. 2 of an embodiment of the tappet similar in many respects to that of'Fig. l; and

Fig. 4 is an axial section to reduced scale of an embodimentof the lower piston of the Fig. 3 tappet differing only in the location and orientation of the vent hole.

In the construction shown in Fig. l, a cylindrical tappet body or shell 2i) is in the form of a cup having an axial recess or bore extending from its upper end arent Q riice 2. toward its lower closed end 2, against which an actuating cam 3 of proper design bears, so that rotation of the cam shaft 3a alternately lifts and lowers the tappet. Such ytappet body 20 1s mounted reciprocatively in a fixture 4, integral with or secured to the engine block, and through which the tappet passes, being held and guided thereby in its reciprocating movements. Such xture al may have oil carrying passages or galleries, indicated at5, furnishing oil from the lubricating oil of the engine to the tappet, the oil being pumped by the oil pump of the engine through the gallery S to a somewhat elongated `and shallow annular chamber 6 made by cutting a wide shallow annular groove around the tappet body or shell Ztl. Such chamber 6 is of a suflicient length that during -all movements ofthe tappet the chamber 6 may receive oil through the passage or gallery 5.

In the lower portion of the shell Ztl a lower plunger piston 22 is slidably mounted for axial movements lengthwise of the shell and is closely tted therein with slight clearance. Ahydraulic compression chamber 2l is dened below the piston 22 which is vertically movable within a cylindrical constriction of the shell bore provided by an annular guide rib or land 23. The diameter ofthe bore portion forming the side wall of chamber 21 is enlarged over the diameter of the bore at the rib 23, but has a smaller reduced coaxial recess or counterbore 24 at the bottom which receives the lower end of fa spring 25. The opposite or upper end of the spring 2S bears against the lower side of piston 22, and preferably encircles a locating lboss 22a projecting downwardly from the piston.

The piston 22 has a vertical axially-disposed passage 26 communicating at its far side or upper end with a valve seat 27 of a conical of funnel form, the sides of which are inclined downwardly and inwardly toward the vertical axis of the piston 22, as shown. The bore of the tappet body or shell 20 above the lannular rib or land 23 may be enlarged at 28 to be of the same inner diam.- eter as is the compression chamber 2l. The bore of the tappet `body or shell 20 above the enlarged portion 28 is constricted or of reduced diameter -to form at29 a secondl annular land, as shown, and may conveniently have the same diameter as the rib or land 23. The upper land 29 serves as a holding and guiding means of cylinder Wall for an upper slid-able member in the nature of a piston 30, just as land 23 does with respect to piston 22, and these -two pistons with the intervening bore Wall deiine between them an upper oil receiving chamber 31.

The piston 30 has a downwardly-extending, axiallylocated solid valve part 32 of a diameter such that its lower end enters the upper end of the funnel valve seat 27. The lower end of the valve 32 may have somewhat rounded corner portions adapted to engage the inwardly inclined sides of the valve seat 27 and the valve is selfcentering and may have approximately a line contact with the seat thereby to enhance the ability to crush completely any foreign particles which may otherwise lodge at the normally contacting surfaces of the valve member and its seat. A coil spring 33 has its lower end seated against the upper side or end of the piston 22, which preferably has an upwardly extending, reduced diameter, coaxial boss portion 2217 around which the spring engages, and the upper end of the spring engages `against the lower face of the upper piston 30 which has a similar coaxial boss 30h for locating the spring.

The upper piston 30 has seated thereon or bearing against it a rod member 34 which may comprise the lower end of a valve stem or other push or lift rod member in the mechanical linkage connections to the normal engine valve. The lower end of this rod 34 is enlarged, as shown, into a head 35 the lower face of which is-aI segment of a sphere which seats in and bears against a suitable complementary recess in the upper side of the upper piston 30.

Oil may be supplied to the upper supply chamber 31 in any suitable manner as, for example, through the gallery 5, annular passage 6 and the side opening 10. As an alternative method of supplying the oil, the upper piston 30 may be provided with an axial passage 36 communicating through a lateral passage 37 with the oil supply chamber 31. Such a construction contemplates that oil normally supplied to the overhead valve operating mechanism in accordance with a well known arrangement will fiow down through a bore in the push rod 34 into the passage 36. The particular manner of directing or supplying the oil to the supply chamber 31 in the tappet is a matter of selection dependent upon circumstances including the general design of the engine'.

In the form of Fig. l the lower end of spring 33 bears directly against the upper surface or side of the piston 22, as shown. Accordingly the net spring force acting on the piston or valve seat member 22 is the resultant or difference between the force of upper spring 33 and lower spring 25. For appropriate action of the tappet under all conditions of operation the spring 33 is made lighter than spring 25 and the spring means acting on the piston 22 is designed in relation to the effective top surface area of the piston to have a net effect of biasing the piston away or up from the closed end 2 of the tappet body or shell toward the valve member 32 with a force less than the atmospheric pressure exertable upon the upper end of the piston 22. The net effect of the spring means in the case of the tappet of Fig. 1

is, of course, the force of spring 25 less the force of spring 33. The supply of oil to the upper side of the piston 22 is frequently arranged to be under pressure which would permit a corresponding increase in the force of spring 25 but the conditions described enable the tappet to operate with complete reliability under all conditions, that is including in particular where the oil is supplied under substantially atmospheric conditions or where the pressure in a pressure supply system falls off to substantially atmospheric pressure.

In operation of the Fig. l embodiment, assume that the cam .3 has been rotated to elevate the engine valve connected to stem 34. After the high point or nose of the cam has passed, the engine valve spring will cause the tappet to move downwardly in contact with the cam until the engine valve comes against its seat. If this occurs before the tappet reaches its lowermost position permitted by the cam, spring 33 will cause the upper piston to remain against the head 35 of the stem 34 with a momentary separation of the lower end of valve 32 from its seat 27, due to at least atmospheric pressure against the upper end of piston 22, and oil will flow from supply chamber 31 into the compression chamber 21 through the momentary opening between the valve and its seat,

to supply additional oil to the compression chamber which is momentarily under a pressure less than atmospheric. The net spring force on the piston 22 tend ing to move its seat 27 upwardly into engagement with valve 32 is less than atmospheric pressure exertable upon the upper side of piston 22. ln the replenishment of oil in the compression chamber 21 when any is needed, as the piston 22 moves upwardly oil simultaneously flows into the space vacated by the piston to maintain the compression chamber filled until the adjustment is completed by engagement of the valve and seat. There is, therefore, automatic supplying of oil to the compression chamber to take up any decrease in length of the mechanical linkage and to replenish any small amounts which may leak upwardly during the lifting of the engine valve.

An important feature of the present invention is the provision in such a tappet construction of means for venting gaseous medium from the pocket below the valve,

constituting a top portion of the compression chamber Air entrained in the oil supplied to such a tappet and other gaseous medium tends, during prolonged operation, to collect in the upper portion of the compression chamber 21, such as just below the valve, in the form of a spherical or oval bubble. Gas lock may result; such a gas bubble will expand and contract under variable pressures with a corresponding disturbance of the proper operation, such as a spongy, mushy or too springy action in lifting the engine valve with attendant improper timing. Such gaseous medium may escape from the compression chamber 2l to the supply chamber 31 through the very small clearance between the cylindrical surface of piston 2.2 and its cylinder wall provided by annular land 23, if conducted thereto, due to the differential in pressure between the lower and upper margins of annular contact between those cylindrical surfaces. The pressure at this lower margin is that prevailing in the compression chamber 21 and that at this upper margin is that, if any, in the supply chamber 31. Let it be assumed that the pressure in the compression chamber`21 is about 10G lbs./ sq. in. gage and that in supply chamber 31 is approximately Zero gage. There is thus a progressive pressurc drop from to 0 lbs/sq. in. gage in the minute clearance upwardly between the piston 22 and shell bore land wall 23. This same relatively high pressure of 100 lbs/sq. in. gage prevails in the pocket formed by the axial passage 26 and the throat portion ofthe space defined by the funnel seat 27 immediately below valve head 32. To vent the gaseous medium or air which collects in this pocket a small hole 42 is drilled laterally through one side of piston 22 to extend from this pocket to some point in the clearance between this piston and the land 23, as shown. At such point the pressure may be about 35 lbs./ sq. in. gage due to the remarked progressive pressure drop, which provides a pressure differential of about 65 lbs/sq. in. gage between the inner and outer ends of vent hole 42, which assures flow of gaseous medium outward therethrough then to bleed upwardly through the clearance between the piston 22 and land 23 to the juncture of the clearance with the supply chamber 31, for escape through the supply passages leading thereto.

Since the escape area at the outer outlet end of vent hole 42 is the circular space defined by the circumference of the vent hole outlet and the piston 22-land 23 clearance, the smaller the vent hole the better. The relatively mobile gaseous medium will flow through this circular escape area at relatively high velocity due to the differential in pressure, but the much less mobile, viscous oil will have only a slow rate of flow through the small vent hole 42 and its outlet, thus minimizing its loss to the supply chamber 31. The minimum size of the vent hole 42 is a matter of practical formation. The usual limitation of smallness is dictated by danger of drill breakage. Normally, the depth of a hole to be drilled through piston material of steel or the like is about five times the diameter of the drill and thus the minimum diameter of vent hole 42 is practically limited to as large as about onefifth of the thickness of the metal through which it is to be drilled. The outlet of vent hole 42 should form a sharp-edged orifice, not chamfered or diverging, so as to avoid increasing the rate of liow and attendant excessive loss of oil. The coefficient of discharge in Bernoullis velocity equation is smallest for a sharp-edged orifice. The vent hole 42 need not have its inlet located precisely at the top of the gas collecting pocket since the gaseous medium or air collects in the form of a spherical or oval bubble due to surface tension, and furthermore, even if the gas bubble be quite small and above the vent hole inlet a slow ow of oil through the venting duct tends to draw the gas bubble over to the inlet and suck it out.

As illustrated in Fig. 2, the gas venting feature may be embodied in a modified form of the tappet wherein tappet body or cup-like shell has only one annular land 123 therein in which is reciprocatively mounted piston 51 with with the, valve member.

which it hasta relatively long bearing,.as shown. This is attained by appreciably lengthening the piston 51l with the formation in the bottom end thereof of a socket, shown at 54, which receives the upperl end of compression spring 12 mounted in compression chamber 9 and resting at its lower end upon the closed end 2 of the shell, normally to move the plunger piston 51Vupwardly when freeto do so. Such spring 12 is of a strength such that when it is compressed to the greatest extent that it can be, its lifting force upon the piston 51 is less than atmospheric pressure upon the upper end of such plunger, The upper end of piston 51 is provided with a relatively wide-mouthed and deep funnel valve seat 127 which communicates by way of a relatively short axial passage 53 with the spring socket.

In the upper end of the tappet body or shell 1211 is a valve member having an upper piston or plunger portion 113 of cylindrical form which closely tits the shell bore for sliding motion up and down therein. This shell bore is of a greater diameter than land 123 so that the upper end of the latter provides anannular shoulder 128, From the lower portion of the piston 113 needle valve 132 extends to contact of its lower end with seat 127 of the piston plunger 51, closing the upper end of the axial passage 53 leading to the compressionchamber 9.

The valve member is normally moved upwardly by a coiled compression spring 16 beneath the lower end of the piston plunger 113 and around the needle valve within oil supply chamber 131 which is dened between pistons 51 and 113. Spring 1 6 is seated upon annular shoulder 128, and the lower end of an engine valve stern or of` a push rod 134 for operating an engine intake or exhaust valve seats upon piston plunger 113. Oil is supplied to the supply chamber 131 through a duct in the push rod 134, an axial bore 50 and a branch duct 137 in the piston plunger 113.

The net. effect of the upward biasingof the piston 51 is to apply a force thereto tending to move it upwardly toward the valve 132 for: closure' of the passage 53 with a force less than the effect of atmospheric pressure exert able on the upper end or oil supply side of this piston, whereby the pressure in the, compression chamber 9 never falls below atmopheric pressure and thischamber remains lled with oil. Thus, spring 12 is of a strength such that when it is compressed to the greatest extent that it can be, its lifting force upon the piston plunger 51 is less than atmospheric pressure upon the upper end of such plunger.

In operation of the Fig. 2, embodiment in an-operating engine, when the cam 3 lifts the tappet 120 with the passage 53 closed by the Valve 132 and a body of practically incompressible liquid, such as oil, filling and trapped in compression chamber 9 the engine valve is lifted by push rod 134, and thereaftery with passage of the cam nose the tappet is lowered by the engine valve spring. Disregarding any mushy or spongy action which will be attendant on a bubble of accumulated gas in the compression charnber, or assuming such has been eliminated by provision of the venting means of the present invention, there is no appreciable downward movement or yielding of plunger S1 other than that which may attend slight seepage of oil through the small clearance between this plunger. and its guide land 123. Due to such seepage the plunger 51 may, at the end of a cycle of lifting and lowering of the tappet 120, be in a slightly lowered position withtseating of the engine valve and consequential removal ofthe load of it and its springV from the tappet assembly while spring 16 lifts the valve piston 113 to contact of push rod 134. As a result, the valve will be cracked, i'. e., Valve seat 127 will be lowered away from the valve member132 to open passage S3 to the oil supply chamber 131', since the light strength of spring 12 biases piston 51" upwardly with a force less than the effect of atmospheric pressure down on this piston and is thus not permitted totraise the latter With downward passage of o il into the compression chamber 9 the fluid pressures on oppositesides or at opposite ends ofpiston 5 1 become balanced and relatively light spring 12` raises the latter to contact of the valve seat 127 by the valve member 132', thereby closing passage 53, and all slack in the engine valve operating mechanism becomes eliminated auto matically.

For the purpose of venting gaseous medium which may accumulate in the upper portion of the compression chamber 9 piston 51 is provided with a small radial passage 52 leading from its axial passage 53 to the circumferential surface thereof as shown in Fig. 2. Any air or other gas which tends to collect in the form of a bubble below the valve, and which cannot escape effectively when the valve is opened due to the inrush of oil will be effectively bled out through the venting duct 52 to the clearance between the piston 51 and its guiding land 123, thence to the supply chamber 131 for ultimate escape, under the influence of pressure differentials existing between the inlet to venting duct` and its outlet and that between the latter and the supply chamber. There is, of course, the previously remarkedprogressive pressure drop around the piston 51 from the lower end upwardly and by suitably. locating the vent hole 52 so as to have a lower pressure at its outer end the gas will be bled off. This ventfeature is particularly advantageous where the piston has a relatively long bearing, as in the Fig. 2 embodiment, so that gas does not readily escape upwardly at its periphery and where there is a pocket, such as at 54, in which gas may collect as in the case where the lower spring 12 is recessed in the bottom of the piston 51 as shown. The sizeof the vent hole 52 is desirably very small within the range of less than about one-sixteenth of an inch,

Fig. 3 showsA the same feature embodied in a tappet having a spring and piston arrangement like that of Fig. 1. In other words, the upper spring 33 bears at its` bottom end'against the top end of lower piston 61 as in the Fig. l construction. Also in Fig. 3 the piston 61 is extended downwardly further and has a longer guiding length as in the Fig. 2 construction. Accordingly, the lower end is recessed at 63 to accommodate the upper end of lower spring 125, resulting in a construction which would permit under some conditions a bubble of gas to collect in the vicinity of passage 126 beneath the valve 32. The bleed opening 62, however, permits the slow escape of such gas from the compression chamber 121. The strength of the lower spring is selected to have the same relation to the piston area and the strength of the upper spring as described in connection with Fig. l, and in general the operation is the same.

It will be noted that in the Fig. 2 construction the outlet of venting duct 52 to the clearance between piston 51 and its guiding land 123 is depicted as located about midway between the bottom of that clearance where the pressure is that of the compression chamber 9 and at the maximum and the top of this clearance or juncture of the latter with the supply chamber 131 Where the pressure is that of the supply chamber and at the minimum. Thus, the pressure. differential between the bleeding duct 52 inlet in the pocket below the valve and its outlet at the clearance is about one half of that between the two chambers, as-determined by the formula- P minus FXP where P is the differential pressure between the two chambers, and F is the ratio between the distance along the clearance from the duct outlet to the juncture and the total length of the clearance between the two chambers. This lower pressure differential may well insure such a slow bleeding of the entrapped gaseous medium or air as to assure a satisfactory minimum of attendant oil loss. On the other hand, the amount of oil loss through the ventingductY 62 of the Fig. 3 construction may be considered to be under certain operating conditions too high since the duct outlet communicates with the clearance much nearer the communication of the clearance with the supply chamber 31 with attendant higher pressure differential. The amount of such oil loss may be reduced inthe Fig. 3 construction by lowering the-,venting duct outlet as proposed in Fig. 4. As shown in Fig. 4, the venting hole 162 might slope downwardly to reduce the pressure differential between its inlet and outlet. However, such orientation of the venting duct has been found not to be essential to practical operation of many embodiments of the tappet of the present invention, especially larger is its outlet orifice and the higher may be the velocity of ow of oil through it resulting in an increase in the amount of oil loss. The characteristic conditions of a particular in:tallation will dictate to one skilled in the art a practical solution within the teachings of the present disclosure.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specic features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a hydraulic length 'compensating mechanism, a cylindrical body provided with a longitudinal bore, said body being closed at one end, a piston reciprocable in the bore and defining a hydraulic compression chamber between one side of the piston and the closed end of said body, means providing a liquid supply chamber communicating with the opposite side of said piston, said piston having a liquid passage therethrough, a valve arranged to control the iiow of liquid through said passage, spring means acting on said valve to bias it away from said piston passage, and spring means acting on said piston adapted to bias the piston away from said closed end toward said valve having a net effect less than the effect of atmospheric pressure exertable on the said opposite side of said piston, said valve being adapted to transmit thrust from means engageable therewith to the piston, said piston having a centrally located space of reduced diameter and area in which gas may collect, said space having a vent duct of small cross section extending to and having its outlet opening in the periphery of the piston at a substantial distance from its compression chamber side.

2. In a hydraulic length compensating mechanism, the combination with a fixture having a bore, of a tappet reciprocatively mounted in the fixture bore and comprising a shell having a cylindrical bore closed at one end and defining an annular piston contact zone of appreciable width having inner and outer margins, a reciprocable piston means slidably mounted in the contact zone of said bore with small clearance therebetween and defining between it and the closed end of said shell bore a hydraulic compression chamber, said shell being imperforate in the annular contact zone thereof, said piston means having a flow passage extending therethrough 'from the hydraulic compression chamber end thereof to its opposite outer end, thrust-transmitting valve means mounted in said shell bore beyond the outer end of said piston means to control flow through said passage, means to supply liquid to the shell bore outward of said annular piston contact zone and consequentially to said outer end of said piston, resilient means biasing said piston means and said valve means outward away from the closed end of said shell bore and a small gas-bleeding vent duct in said piston leading from said compression chamber to and terminating in the outer peripheral surface of said piston at a point between inner and outer margins of said contact zone for communication to said clearance and thence by way of the latter to the liquid supply at the outer end of said piston.

3. The hydraulic length compensating mechanism as defined in claim 2 characterized by said liquid supply means comprising a supply chamber in said shell defined on one side by said outer end of said piston,.and said vent duct communicating with said passage inward of said valve means and terminating in an outlet port which during reciprocation of sai-d piston means is opposed at all times to the imperforate bore wall in said contact zone whereby gas bled therefrom passes outward through the clearance between the contact zone bore Wall and said piston means to said supply chamber along a path appreciably shorter than that provided between said compression and supply chambers by way of the clearance between said contact zone bore wall and said piston means in the absence of said duct.

4. In a hydraulic length compensating mechanism a. tappet device comprising, in combination, a shell adapted for up and down reciprocation and having a cylindrical bore closed at its bottom end, the lower portion of said bore defining an imperforate annular piston contact zone having upper and lower margins of sliding contact with said piston, a reciprocable cylindrical piston slidably mounted in said bore contact zone with slight clearance between the cylindrical surface of said piston and the imperforate Wall of said contact zone and having upper and lower ends, the lower end of said piston defining with said closed bottom end of said bore a hydraulic co-mpression chamber below said piston, said piston having a fiow passage extending therethrough between its upper and l-ower ends for supply of liquid to said compression chamber, a valve plunger constituting a thrust-transmitting member mounted in the top of said shell bore defining between it and said piston a liquid supply chamber, means to supply liquid to said supply chamber, a valve member on said plunger to close said passage, a spring in said compression chamber biasing said piston upwardly, another spring in said supply chamber biasing said plunger upwardly, and a vent meansv comprising a single small gas-bleeding vent duct provided through .one side of said piston leading from said passage to the cylindrical surface of said piston and terminating in an outlet at the clearance located between said upper and lower margins of said imperforate contact zone an appreciable distance above the lower margin where there is a substantial pressure differential between the compression chamber pressure in said passage and reduced pressure in said clearance, said duct outlet being located an appreciable distance from the juncture of said clearance with said supply chamber to minimize oil loss with vent of gas successively through said duct and clearance.

5. The hydraulic length compensating mechanism as defined in claim 4 characterized by the outlet of said vent duct being in the form of a sharp-edged orifice.

6. In a hydraulic length compensating mechanism, the combination with a fixture hav-ing a bore, of a tappet reciprocatively mounted in the fixture bore and comprising a shell having a cylindrical bore closed at one end and defining an annular piston contact zone of appreciable width having inner and outer margins, a reciprocable piston means slidably mounted in the contact zone of said bore with small clearance therebetween and defining between it and the closed end of said shell bore a hydrau- Iic compression chamber, said shell being imperforate in the annular contact zone thereof, said piston means having a flow passage extending therethrough from the hydraulic compression chamber end thereof to its opposite outer end, valve means mounted in said shell bore beyond the outer end of said piston means to control flow through said passage, means to supply liquid to the shell bore outward of said annular piston contact zone and consequentially to said outer end of said piston, resilient means biasing said piston means outward away from the closed end of said shell bore, means to transmit thrust from said piston means and a small gas-bleeding vent duct in said piston leading from said compression chamber to and terminating in the surface comprising said contact zone at the periphery of said piston at a point between inner and outer margins of said contact zone for communication to said clearance and thence by way of the latter to the liquid supply at the outer end of said piston.

7. In a hydraulic length compensating mechanism adapted to be embodied in a linkage for operating a valve, a cylindrical body adapted to be mounted for axial movement provided with a longitudinal bore, said body being closed at the bottom end, a piston reciprocable in the bore and defining a hydraulic compression chamber between said piston and the closed end of said body, means providing a source of supply of liquid for said compression chamber, a connecting uid passage and valve means for controlling the supply of liquid from said source to said compression chamber, spring means for urging said piston away from said closed end, means for transmitting thrust to said piston in opposition to said spring and adapted to compress the liquid in said chamber, said piston having a peripheral surface providing an annular contact zone with the` interior of said body of considerable width longitudinally of said body, said piston having a centrally located pocket in communication with said compression chamber of reduced diameter and area permitting the collection of gas therein and a vent means therefor comprising a single small gas bleeding duct leading from said pocket and having its termination in said peripheral surface at a point spaced a considerable proportion of the Width of said contact zone from the bottom edge thereof and providing a pressure intermediate that below the piston when under compression and that above the piston.

References Cited in the le of this patent UNITED STATES PATENTS 1,936,653 Almen Nov. 28, 1933 2,090,795 Johnson 2--- Aug. 24, 1937 2,140,826 Bettison Dec. 20, 1938 2,267,535 Osborn Dec. 23, 1941 2,541,953 Banker Feb. 13, 1951 

